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AC / DC: What's the Difference?

There is more than one way to produce electricity. In the late 1800s, Thomas Edison's direct current power plants prevailed. Shortly after, however, alternating current took hold. This animated essay from the American Experience:Edison's Miracle of Light website describes and illustrates the similarities and differences between these two forms of electricity.

In the late 1800s, more than a hundred Edison power stations provided electricity to residents in dozens of communities in the United States. These stations produced direct current (DC) electricity, the standard for power at the time, and the type of electricity still produced by batteries today. But Edison's plants had serious limitations. Much of the power they produced was lost as heat during transmission, because it met resistance while traveling through the wires.

Inefficiency in electrical systems is usually the result of such heat loss. The more current that flows through a system, the greater the energy loss will be. The only way to reduce current and maintain the same power output is to increase voltage, the push that causes charges to move through a circuit. This push is caused by the difference in potential electric charge between the generator and the customer. The greater the voltage difference, the greater the push, and the more efficient power transmission will be.

Transmitting power at high voltage requires a mechanism to first raise voltage to high transmission levels and then bring it back down for household use.

Direct current electricity flows in only one direction, and voltage is determined at its source and cannot be changed. In contrast, alternating current (AC) electricity changes direction many times each second. Devices called transformers can use this change of direction to increase or decrease the voltage of electricity flowing through them. Some of the largest transformers can raise voltage to as high as 700,000 volts for efficient transmission. Transformers on the receiving end then step the voltage back down to the 110 or 220 volts that most household appliances require.

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